This invention relates to a molding machine such as an injection molding machine or a die cast machine and more particularly to the improvement of the mold tightening apparatus utilized in such molding machine.
With the recent trend of increasing the capacity of the molding machine of the type described above, and for the purpose of decreasing the weight and floor space occupied by the machine as well as the price and operating cost thereof, decreasing the size of the liquid pressure actuator and increasing the operating speed of the machine, it has been proposed to use two operating cylinders, namely a boost cylinder for moving the movable die plate for opening and closing the mold, and a mold tightening cylinder for strongly urging the movable mold against the stationary mold instead of using a single cylinder for moving the die plate and tightening the molds.
FIG. 1 of the accompanying drawing diagrammatically illustrates one example of such a two cylinder type molding machine comprising a stationary metal mold 1 secured to a stationary die plate 2 securely mounted on a bed, not shown, a stationary plate 3 also secured to the bed, and tie bars 4 extending between plates 2 and 3. The lefthand ends of the tie bars 4 slidably extend through the openings of the stationary plates 3, whereas the righthand ends slidably extends through openings of the stationary die plate 2 and are terminated with pistons 5 contained in cylinders 6 fastened to the stationary die plate 2. The lefthand halves of tie bars 4 are formed with screw threads or a plurality of axially spaced circumferential grooves. There is also provided a movable metal mold 7 supported by a movable die plate 8 for cooperation with stationary mold 1. The movement of the movable die plate 8 is guided by tie bars 4. Locking members in the form of split lock nuts 9 formed on their inner surfaces with threads or axially spaced circumferential ribs adapted to engage the threads or grooves of the tie bars 4 are mounted on one side of the movable die plates 8 to be slidable in the radial direction with respect to the longitudinal axes of the tie bars 4. Thus, when the locking members are caused to engage or clamp the tie bars, the movable die plates 8 and tie bars 4 are locked together so that they are moved in unison. Above one of the cylinders 6 is mounted a boost cylinder 10 whose piston rod 11 is connected to the movable die plate 8. With this construction, when pressurized liquid is admitted into the lefthand chamber in the boost cylinder 10 while the stationary and movable molds 1 and 7 are separated the movable die plate 8 will be moved to the right along the tie bars. At the end of the stroke of the piston in boost cylinder 10, the locking members 9 are moved toward tie bars 4, in a manner to be described later, thus locking the movable die plate 8 to the tie bars 4. Then, the pressurized liquid is admitted into the lefthand chamber of mold tightening cylinder 6 for urging the movable mold 7 against the stationary mold 1 under a high pressure.
With this arrangement, however, as the movable die plate 8 is locked to the tie bars by advancing radially inwardly the locking members 9 when the stroke of the piston in the boost cylinder 10 has completed, the time required for advancing the locking members causes in the decrease in the production speed.